L.S.U. running back Leonard Fournette, a junior, hurdling an Auburn player last month. Had Fournette been allowed to enter the draft after his sophomore year, he would most likely have been a high pick.CreditButch Dill/Associated Press

TUSCALOOSA, Ala. — Midseason is no time for eulogies — still less from the coach of a team that has just won an important game. Yet Alabama Coach Nick Saban was distinctly funereal at one point after the Crimson Tide’s 33-14 win over Texas A&M on Saturday night.

“On a down note, Eddie Jackson has a fractured leg, and he will be out for the season,” Saban said, referring to his starting safety, who left the game in the fourth quarter after being tackled while returning a punt.

Saban added, “We’re certainly going to miss him, but what a great player, what a great competitor and what a great guy to have in this program for the time he’s been here.”

The past tense was appropriate because Jackson is a senior, in his fourth and final year of eligibility. Widelyforecast as a first-round pick at the beginning of the season, he will now enter the N.F.L. draft.

A few hours later, and two states over, Leonard Fournette was making his own case for a high draft slot. Fournette, a Louisiana State junior running back who has been celebrated since his high school days in New Orleans, gashed Mississippi for 284 yards and three touchdowns on only 16 carries.

In different ways, the players’ experiences Saturday shed light on the awkward and dangerous aerobics that N.F.L. eligibility requirements, N.C.A.A. amateurism rules and the nature of football itself impose on top prospects.

This sort of quandary is much discussed in basketball. For a decade, the N.B.A. has required players to be one year removed from high school before they can enter the draft. The resulting one-and-done phenomenon, while reshaping the college game, has raised questions of fairness for 18-year-olds talented enough to be drafted. Some young prospects have even spent a year abroad — where they can play for a salary — and then entered the draft, leading to further debate.

But football is not the same as basketball, or really any other sport. The N.F.L., which requires players to have been out of high school for at least three years, is so physically, intellectually and emotionally demanding that few observers make a case for including even the most gifted teenagers. And unlike baseball or hockey, football has not spawned a viable farm system other than the college game.

So promising players essentially must play in college for three years, for no compensation beyond a scholarship, while risking a sprained wrist, a broken leg or even a traumatic head injury — injuries that could slightly or severely damage their future prospects. (And injury insurance policies have proved sadly inadequate.)

The decision, then, becomes whether to return for a fourth year — by which time players, who typically take classes year-round, may have already received a degree — and be subjected to the same risks in the service of a somewhat higher draft slot.

“You hate when young people get injured,” said Todd Berry, the executive director of the American Football Coaches Association. But, he added in reference to Jackson and other players like him, “even ones who have been injured have tended to raise their stock some” by playing college football for another season.

Gil Brandt, a former player personnel executive for the Dallas Cowboys who now writes for NFL.com, said of players who return for their final season of eligibility: “They’re going to be better football players. I think if you’re a better football player, you get drafted higher. If you have experience and maturity, you end up being a better player quicker.”

That seems true. Jackson could have turned pro, and perhaps even been a first-round pick, after last season, which he finished by being named the national championship game’s most valuable defensive player. But he calculated that he had a better chance of securing an even higher draft position, and the greater money that would come with it under the N.F.L.’s strictly slotted system for rookie contracts, by staying in school.

Now, though, after his injury, teams might think twice.

“I don’t think his broken leg is going to hurt him in the draft,” Brandt said. “It’s unfortunate that he didn’t have an opportunity to get more accolades, because he probably would have been first-team all-American. I’d just tell him I thought he did a great job staying in school.”

According to The Associated Press, Jackson had surgery to repair his leg on Tuesday.

Saban said that the operation “went really, really well.”

Had Fournette been allowed to enter the draft after his sophomore year, he would most likely have been a high pick. Even Brandt acknowledged that Fournette, who moves like a lightning bolt and stings like a punch to the jaw, should “think strongly about coming out” — that is, he should enter the draft next year, leaving a season of college eligibility on the table. (He almost certainly will.)

The calculus is different for running backs, Brandt said. They have notoriously short shelf lives, in part because of the beatings they take in the course of play, and the relatively simple responsibilities of the position — take ball, see the hole, run through the hole — mean they do not need the kind of extensive tutelage that quarterbacks, offensive linemen and safeties do.

Even this season poses a risk for Fournette. That sort of jeopardy similarly troubled South Carolina defensive end Jadeveon Clowney, who publicly flirted with sitting out his junior year in 2013. He eventually played for the Gamecocks, and although he sustained a few minor injuries, he was drafted No. 1 over all in 2014.

Brandt said that for someone in Fournette’s position, the bigger risk would be not to play.

“I would think they would wonder why he didn’t want to compete,” Brandt said of N.F.L. front offices. “I’d say it’s all part of the game, and the more you can prove yourself to me as a player, the more likely I am to pick you.”

Berry, of the coaches’ association, suggested that changing the rules because of an individual player like Fournette would be an instance of shaping policy around an outlier. “I think we always have to look at the majority,” Berry said. “There are exceptions. We have to protect the other ones, to make sure we serve their interests.”

It is difficult to conceive of what could change to make life easier on the Eddie Jacksons and Leonard Fournettes. Football’s mandatory experience and maturity, limited presence outside North America and inherent hazards are all encoded in its DNA.

It might be interesting to see how incentives would realign if players could be paid while in college. But that rule, too, seems unlikely to change. And Jackson and Fournette, at least, will have to wait several months for their first salaries.

A version of this article appears in print on , on Page B12 of the New York edition with the headline: Top Prospects Walk a Perilous Tightrope for N.F.L. Eligibility. Order Reprints | Today’s Paper | Subscribe